The present invention relates to flexible, UV curable, epoxysilicone-polyether coatings which are obtained by the incorporation of polyether block segments into linear SiH-containing silicone backbones, which may be followed by conversion of the SiH-containing silicone-polyethers into their epoxy-containing linear block copolymer derivatives. The invention also relates to flexible, UV-curable epoxysilicone-polyether coating as described above and further incorporating one or more fluorescent dye markers. The product of the present invention is useful for a wide range of coating applications including release coatings, optical fiber buffer coatings, conformal coatings and electronic encapsulation, and when produced with UV-detectable dye marker, the product of the invention is particularly useful for ascertaining the integrity of very thin coatings The invention further relates to a process for producing the above-mentioned epoxy-containing silicone-polyether linear block copolymers.
Epoxysilicone polymers have been widely used in the release coating and pressure-sensitive adhesive (PSA) industries. For example, see generally the chapter entitled "Silicones" by B. Hardman and A. Torkelson in the Encyclopedia of Polymer Science and Engineering, 2nd edit., Vol. 15, pp. 204-308, 1989, John Wiley & Sons, Inc., New York. Epoxysilicone polymers are conveniently manufactured through the hydrosilation reaction between an SiH-containing silicone monomer or polymer and olefin epoxides. The general hydrosilation reaction between a silicone and olefin can be expressed for monofunctional silane derivatives as EQU .tbd.SiH+CH.sub.2 .dbd.CH-Q.fwdarw..tbd.Si--CH.sub.2 --CH.sub.2 --Q
and for di-functional siloxane derivatives as EQU --(--CH.sub.3 (H)SiO--)--+CH.sub.2 =CH--Q.fwdarw.--(--(CH.sub.3) (QCH.sub.2 CH.sub.2) SiO--)--
where in both cases Q is an organic radical. The hydrosilation reaction is particularly useful for the addition of functional radicals onto silanes and silicones. For example, reaction of a hydrogensiloxane with an epoxy-containing olefin yields an epoxy-functional siloxane.
Epoxysiloxanes generated through, for example, the hydrosilation reaction can be cured either thermally or, in the presence of the appropriate catalysts and possibly accelerators, by irradiation. Generally, UV-induced, cationic catalysis is preferred in the cure reaction of epoxysilicones due to the relatively low cost of this process, relatively high cure rates achieved, the low temperature which can be employed, thereby preventing damage to temperature-sensitive materials being coated, and the low risk of potential hazards to both industrial users and the environment. Upon exposure to UV radiation, cationic type photo-initiators generate a strong Bronsted acid, which effects the opening of the oxirane ring in the epoxide radical of an epoxysilicone polymer, and the subsequent etherification through which cross-linking of the resin is achieved.
The curing of epoxysilicone polymers is well documented in the patent literature. For example, U.S. Pat. No. 4,576,999, issued to Eckberg, discloses epoxy and/or acrylic functional polysiloxanes as UV-curable abhesive release coatings. The catalyst may be a photo-initiating onium salt and/or a free radical photo-initiating catalyst U.S. Pat. Nos. 4,279,717 and 4,421,904, both issued to Eckberg, et al., disclose epoxy functional diorganosiloxane fluids combined with iodonium salts to form UV-curable abhesive release compositions. U.S. Pat. No. 4,547,431 discloses epoxy functional diorganosiloxane combined with onium salt catalyst and polyfunctional epoxy monomers to also form an abhesive release coating. All patents and publications mentioned herein are incorporated by such reference.
As described in U.S. Pat. No. 4,576,999, the preferred UV photo-initiators for the curing of epoxysilicones are the "onium" salts, of the general formulas EQU R.sub.2 I.sup.+ MX.sub.n.sup.- EQU R.sub.3 S.sup.+ MX.sub.n.sup.- EQU R.sub.3 Se.sup.+ MX.sub.n.sup.- EQU R.sub.4 P.sup.+ MX.sub.n.sup.- EQU R.sub.4 N.sup.+ MX.sub.n.sup.-
where different radicals represented by R can be the same or different organic radicals from 1 to about 30 carbon atoms, including aromatic carbocyclic radicals from 6 to 20 carbon atoms which can be substituted with from 1 to 4 monovalent radicals selected from C.sub.(1-8) alkoxyl, C.sub.(1-8) alkyl, nitro, chloro, bromo, cyano, carboxy, mercapto, etc., and also including aromatic heterocyclic radicals including, for example, pyridyl, thiopheny, pyranyl, and others; and MX.sub.n.sup.- is a non-basic, non-nucleophilic anion, such as BF.sub.4.sup.-, PF.sub.6.sup.-, AsF.sub.6.sup.-, SbF.sub.6.sup.-, HSO.sub.4.sup.-, ClO.sub.4.sup.-, and others as known in the art. The photo-initiators may be mono- or multi-substituted mono, bis or tris aryl salts. In the above and subsequent definitions, the prefix "hetero" is meant to include linear or cyclic organic radicals having incorporated therein at least one non-carbon and non-hydrogen atom, and is not meant to be limited to the specific examples contained herein. According to U.S. Pat. No. 4,977,198, the onium salts are well known, particularly for use in catalyzing cure of epoxy functional materials.
As disclosed in U.S. Pat. No. 4,882,201, the radiation-initiated cure of epoxysilicones coated on a substrate can be achieved with UV lamps such as: mercury arc lamps (high, medium and low pressure), Xenon arc lamps, high intensity halogentungsten arc lamps, microwave driven arc lamps and lasers. Additionally, ionizing radiation using, for example, .sup.60 Co is also useful as a radiation source. In this latter instance, the ionizing radiation serves both to initiate cure and at the same time sterilize an epoxysilicone coating.
Certain polyether-silicone copolymers are known. For example, U.S. Pat. No. 4,988,504 discloses polysiloxane polymers bearing pendant polyether radicals for use in stabilizing silicone emulsions. Similarly, Japanese Published Patent Application 02-129219 discloses use of epoxysilicones bearing radially pendant polyethers, described as having good compatibility with onium salt photo-initiators, for use as coatings which can be printed on. U.S. Pat. Nos. 4,758,646 and 4,859,529 disclose bis(alkoxysilyl)polyethers for use as a fabric sizing agent, and U.S. Pat. No 4,184,004 describes organosilicone terpolymers bearing radially pendant polyethers as a fabric softening agent. In each of the above-mentioned patents, the polymer may be described as a "block" copolymer with respect to the silicone monomeric units. For example, compounds of the formula EQU --(--(CH.sub.3).sub.2 SiO).sub.x -(Q.sub.2 SiO).sub.n -((CH.sub.3).sub.2 SiO--).sub.y --
wherein n, x and y are greater than 1 and the substituent Q (for example, a polyether) is in a radially pendent block, as opposed to randomly distributed, with respect to the (CH.sub.3).sub.2 SiO backbone. This molecular organization should be contrasted with that of a "linear block copolymer" of the general formula EQU --(--(CH.sub.3).sub.2 SiO).sub.x -(Q).sub.n -((CH.sub.3).sub.2 SiO--).sub.y --
wherein n, x and y are greater than 1 and the substituent Q again perhaps a polyether monomeric unit, is incorporated directly into a linear siloxane backbone.
Dye sensitizers are also known as, for example, disclosed in the aforementioned U.S. Pat. No. 4,977,198. Dye sensitizers are cure accelerators which serve to increase the effectiveness of the photocatalyst by generally absorbing light that is of a wavelength outside the useful range of that of the photo-initiator and transferring the absorbed energy to the photo-initiator. Thus, the dye sensitizer results in better utilization of the energy available from a light source, with the result that this source need not be specifically tuned to match the main absorption wavelength of the photo-initiator. Dyes which are useful with the above-described onium salts are cationic dyes, such as shown in Vol. 20, pages 194-197 of the Kirk-Othmer Encyclopedia, 2nd Edition, 1965, John Wiley & Sons, New York. Some of the cationic dyes which can be used as sensitizers include, for example,
Acridine Orange; C. I. 46005; PA1 Acridine Yellow; C.I. 46035; PA1 Phosphine R; C.I. 46045; PA1 Benzoflavin; C.I. 46065; and, PA1 Setoflavin; C.I. 49005. PA1 Hematoporphyrin; PA1 4,4'-bisdimethylaminobenzophenone; and, PA1 4,4'-bisdiethylaminobenzophenone.
In addition, some basic dyes can also be used as sensitizers. Some of these basic dues are shown in Vol. 7, p.532-4 of Kirk-Othmer Encyclopedia, as cited above, and include:
Also suitable are xanthones, such as thioxanthone, 2-isopropyl xanthone, and aminoxanthene. Specific instances where dye sensitizers are employed are detailed, for example, in U.S. Pat. No. 4,026,705.
A major drawback to the use of the "onium" salt catalysts in the polymerization of epoxysilicones lies in the highly polar nature of these salts. As the commonly used silicones are based on non-polar polydimethylsiloxane polymers, the polar "onium" catalysts are not sufficiently miscible with the resin to affect as fast a cure rate as would generally be desirable nor are suspensions of the insoluble catalysts stable. The need therefore exists to devise novel materials and processes in which the miscibility of the photo-initiators and siloxanes are much improved.
Two general approaches have been taken to increase the miscibility of an onium photo-initiators and an epoxysilicone resin. The first approach has been to increase the hydrophobicity of the catalyst through use of onium salts containing non-polar, organic radicals. This approach led to investigations of potential onium salts, particularly long-chain alkyl-substituted bisaryliodonium salts, which are less polar in nature than their sulfonium counterparts. As disclosed in U.S. Pat. No. 4,882,201, particularly useful catalysts of this type are the linear or branched, C.sub.8 or greater alkoxy, mono- or disubstituted, bisaryliodonium salts. As further disclosed in U.S. Pat. No. 4,882,201, the long-chain, alkoxy-substituted aryliodonium salts also possess the useful property of being much less toxic than the non-substituted onium salt photo-initiators.
The second approach to alleviating the aforementioned miscibility problem between the photo-initiator and a silicone has been to incorporate silphenylene blocks into a siloxane backbone, for example as disclosed in U.S. Pat. No. 4,990,546. This approach, when coupled with the use of the above-described substituted onium salts, proved useful in increasing photo-initiated cure. However, the incorporation of silphenylene blocks into a silicone resin is not commercially viable, since the disilyl-functional benzenes needed to produce the silphenylene-containing polymers are not available in commercial quantities.
In a more indirect effort to overcome the relatively slow cure rates due to the above-mentioned miscibility problem, an epoxysilicone resin is "pre-crosslinked" as disclosed, for example, in U.S. Pat. No 4,987,158. While such "pre-crosslinked" epoxysilicone networks, formed from vinyl tetramer and SiH-containing linear silicones partially overcome some of the slow cure associated with long chain epoxysilicone coatings, these partially-cured resins still do not possess a solubility with iodonium catalysts that is sufficiently high to be commercially useful as UV-curable materials in most applications.
Another problem typically encountered in the silicone coating industry, is that of the difficulty in assessing the sufficiency of very thin silicone coatings, particularly when these coatings are applied to shiny or glossy types of film liner. For example, in the release coating and electronics industry, if the coating on a substrate contains gaps wherein no silicone is present, there results a poor product performance and the subsequent economic waste associated therewith. It would be advantageous therefore to provide a system whereby the integrity of a silicone release coating could be easily and economically evaluated. In particular, it would be desirable to incorporate a colorless marker into a silicone resin, as this would still allow visually clear coatings to be produced UV-detectable dye markers would be particularly preferable for such use. However, possibly in part due to the above-mentioned miscibility problems encountered with the use of non-polar silicone resins, such a system has heretofore not been available.
Due to the above-mentioned considerations, it has therefore been desirable to search for novel ways in which to increase the miscibility of polar compounds, particularly photo-initiator salts, in epoxysilicone resins such that high and efficient cure rates can be economically achieved. In addition it would also be advantageous to, at the same time, provide for epoxysilicones that are more flexible and elastomeric than traditionally available, and thus have a greater number of potential uses than current resins. Finally, it would be particularly advantageous to, at still the same time, provide a system whereby the integrity of application of a silicone coating, particularly thin clear coatings, on glossy or shiny substrates can be easily monitored.
One aim of the present invention is to alleviate the miscibility problems of onium salt catalysts with silicone resins, and thereby improve the cure characteristics of these resins. A second aim of the invention is to prepare more highly flexible and elastomeric silicone resins, also with faster cure rates than previously possible. Yet another aspect of the present invention is to devise a system for easily and accurately monitoring the integrity of a coating of these resins onto a substrate. A further aim of the present invention is to provide a process for preparing a resin with the above-mentioned characteristics.